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Date:   Tue, 8 Jun 2021 10:45:44 +0200
From:   Jan Kara <jack@suse.cz>
To:     Roman Gushchin <guro@fb.com>
Cc:     Jan Kara <jack@suse.cz>, Tejun Heo <tj@kernel.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, Alexander Viro <viro@zeniv.linux.org.uk>,
        Dennis Zhou <dennis@kernel.org>,
        Dave Chinner <dchinner@redhat.com>, cgroups@vger.kernel.org,
        Jan Kara <jack@suse.com>
Subject: Re: [PATCH v8 3/8] writeback, cgroup: increment isw_nr_in_flight
 before grabbing an inode
Message-ID: <20210608084544.GB5562@quack2.suse.cz>
References: <20210608013123.1088882-1-guro@fb.com>
 <20210608013123.1088882-4-guro@fb.com>
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On Mon 07-06-21 18:31:18, Roman Gushchin wrote:
> isw_nr_in_flight is used do determine whether the inode switch queue
> should be flushed from the umount path. Currently it's increased
> after grabbing an inode and even scheduling the switch work. It means
> the umount path can be walked past cleanup_offline_cgwb() with active
> inode references, which can result in a "Busy inodes after unmount."
> message and use-after-free issues (with inode->i_sb which gets freed).
> 
> Fix it by incrementing isw_nr_in_flight before doing anything with
> the inode and decrementing in the case when switching wasn't scheduled.
> 
> The problem hasn't yet been seen in the real life and was discovered
> by Jan Kara by looking into the code.
> 
> Suggested-by: Jan Kara <jack@suse.com>
> Signed-off-by: Roman Gushchin <guro@fb.com>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza


> ---
>  fs/fs-writeback.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 3564efcc4b78..e2cc860a001b 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -505,6 +505,8 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
>  	if (!isw)
>  		return;
>  
> +	atomic_inc(&isw_nr_in_flight);
> +
>  	/* find and pin the new wb */
>  	rcu_read_lock();
>  	memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys);
> @@ -535,11 +537,10 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
>  	 * Let's continue after I_WB_SWITCH is guaranteed to be visible.
>  	 */
>  	call_rcu(&isw->rcu_head, inode_switch_wbs_rcu_fn);
> -
> -	atomic_inc(&isw_nr_in_flight);
>  	return;
>  
>  out_free:
> +	atomic_dec(&isw_nr_in_flight);
>  	if (isw->new_wb)
>  		wb_put(isw->new_wb);
>  	kfree(isw);
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR